Source: Unknown
A man with a thick, dark mustache is holding a smartphone to his ear, pretending to be on a call. He then looks directly at the camera with a playful expression and places a finger over his lips to gesture for silence or to keep a secret, before returning to his phone.
This video is used as a reaction to indicate that something needs to be kept confidential, a secret is being shared, or to tell someone to quiet down because they are being too loud about private information.